Email glossary: the terms behind email delivery
Email has a jargon of its own, and that jargon shows up at exactly the moment something goes wrong: a message that is deferred, an address that hard bounces, a domain without DKIM. This glossary explains those terms in one sentence each, without needing another article alongside it.
31 terms, grouped from the foundation (authentication) towards practice (list management and technology). Every term has its own anchor, so you can link to it directly.
Authentication & DNS
SPF #
SPF (Sender Policy Framework) is a DNS record in which the owner of a domain states which servers are allowed to send email on that domain's behalf.
The receiving server compares the sending IP address against that list. A domain may only have one SPF record; adding several services by creating a record per service breaks the check entirely.
Detailed explanation →DKIM #
DKIM (DomainKeys Identified Mail) is a digital signature the sending server adds to every outgoing email and that the recipient verifies with a public key published in DNS.
It proves the message was not altered in transit. DKIM is more robust than SPF because it does not depend on an IP address: a forwarded message stays valid, where SPF often fails.
Detailed explanation →DKIM selector #
A DKIM selector is the label in the signature that indicates which DNS record holds the matching public key.
Thanks to the selector, one domain can publish several DKIM keys at the same time — for example one per sending service — and rotate a key without interruption.
DMARC #
DMARC (Domain-based Message Authentication, Reporting and Conformance) is a DNS record that tells recipients what to do with email that fails the SPF and DKIM checks.
The policy is set to p=none (report only), p=quarantine (to the spam folder) or p=reject (refuse). DMARC also requires the visible sender domain to match the domain that SPF or DKIM validated.
Return-Path (envelope sender) #
The Return-Path is the address to which error messages about an email are returned, and it is the address SPF is checked against.
It is separate from the From address the recipient sees. That difference explains why SPF can pass while DMARC still fails: DMARC requires both domains to belong together.
BIMI #
BIMI (Brand Indicators for Message Identification) is a DNS record through which a domain publishes a logo that supporting mailboxes display next to the message.
BIMI only works on top of a DMARC policy set to quarantine or reject. Some providers additionally require a verified brand certificate.
MTA-STS #
MTA-STS (SMTP MTA Strict Transport Security) is a policy through which a domain enforces that incoming email is delivered only over a verified TLS connection.
Without MTA-STS an attacker can strip the encryption in transit. The accompanying TLS-RPT record lets senders report when that happens.
Delivery & reputation
Deliverability #
Deliverability is the share of sent email that actually reaches the recipient's inbox, rather than landing in the spam folder or being refused.
It differs from delivery: a message can be accepted by the receiving server (delivery succeeded) and still sit in the spam folder.
Detailed explanation →IP reputation #
IP reputation is the judgement mailbox providers form about a sending IP address, based on send volume, complaints, bounces and recipient behaviour.
A new or long-unused IP address has no reputation, which is not the same as a good reputation: when in doubt, providers filter more strictly.
Domain reputation #
Domain reputation is the judgement mailbox providers form about the sending domain itself, independently of the IP address it sends from.
Because reputation attaches to the domain, it travels with it to a new sending service. A damaged domain reputation therefore cannot be fixed by switching IP address.
IP warm-up #
IP warm-up is the controlled growth of send volume from a new IP address or domain, so mailbox providers get to know the sender before the volume becomes large.
Without warm-up, a sudden spike from an unknown address reads as a spam outbreak. A warm-up typically runs over several weeks and starts with the most engaged recipients.
Detailed explanation →Dedicated IP and shared IP #
A dedicated IP is a sending IP address used by a single sender; with a shared IP, several senders use the same address and therefore the same reputation.
A dedicated IP gives full control but requires enough regular volume to build and keep a reputation. At low volumes a well-managed shared IP usually performs better.
Greylisting #
Greylisting is a defence in which the receiving server temporarily refuses a first delivery attempt from an unknown sender and only accepts a later attempt.
Real mail servers retry automatically; many spam senders do not. The result is a delay of a few minutes on the first email to such a recipient.
Throttling (rate limiting) #
Throttling is limiting the number of emails sent to one receiving party within a time window.
Mailbox providers apply it to flatten peaks; senders apply it to avoid running into those limits. Exceeding them results in delays or temporary refusals.
Deferral (4xx) #
A deferral is a temporary refusal with a 4xx status code, in which the receiving server asks for the message to be offered again later.
A deferral is not a bounce. If it persists, it is usually a reputation or volume signal rather than a technical problem.
Blocklist (DNSBL) #
A blocklist is a published list of IP addresses or domains flagged as spam sources, which receiving servers consult over DNS.
Not every list carries equal weight: a handful of large lists account for most blocks, while obscure lists are consulted by almost no one.
Spam trap #
A spam trap is an email address that exists purely to catch unsolicited mail and that was never signed up voluntarily.
Besides addresses that never belonged to a real person, long-abandoned mailboxes are also turned into traps. That is why an old, never-cleaned address list is dangerous.
Bounces & list management
Hard bounce #
A hard bounce is a permanent refusal with a 5xx status code, because the address does not exist or the domain does not accept email.
A hard-bouncing address belongs off the list immediately. Continuing to send to non-existent addresses is, to mailbox providers, the clearest signal that a list is not maintained.
Soft bounce #
A soft bounce is a temporary refusal, for example because the mailbox is full or the receiving server is briefly unreachable.
A soft bounce justifies another attempt. If it repeats across several sends, it is better to treat the address as permanently unusable.
Bounce processing #
Bounce processing is the automatic reading of error messages and the disabling or retesting of addresses on that basis.
Without processing, invalid addresses stay on the list and keep generating bounces — with a declining reputation as a result.
Opt-in and double opt-in #
Opt-in is the consent someone gives to receive email; with double opt-in that consent is then confirmed through a link in a verification email.
Double opt-in yields a smaller but demonstrably valid list, and filters out typos and third-party addresses right away.
Detailed explanation →List-Unsubscribe (One-Click) #
List-Unsubscribe is an email header that lets the mailbox itself show an unsubscribe button, without the recipient having to open the email.
The One-Click variant handles the unsubscribe in a single request, with no intermediate page or confirmation. Large providers expect this header on bulk sends.
Spam complaint and feedback loop #
A spam complaint arises when a recipient hits the spam button; through a feedback loop the mailbox provider reports that complaint back to the sender.
With providers, a spam complaint weighs far heavier than an unsubscribe. That is why a hidden unsubscribe link is more damaging than a visible one.
List hygiene #
List hygiene is the maintenance of an address list: removing bounces, processing unsubscribes, merging duplicates and retiring long-inactive recipients.
A smaller, engaged list usually returns more than a large list full of dead addresses — and protects the reputation every send depends on.
Sending & technology
SMTP #
SMTP (Simple Mail Transfer Protocol) is the protocol mail servers use to hand email to each other and applications use to submit their outgoing email.
Because the protocol is universal, any application that speaks SMTP can send through any SMTP service — the settings differ, the protocol does not.
SMTP relay #
An SMTP relay is an intermediate mail server that accepts outgoing email from applications and delivers it to the receiving servers on their behalf.
The relay brings authentication, reputation, retries and logging together in one place, instead of leaving them to each application separately.
Detailed explanation →Port 587 and STARTTLS #
Port 587 is the standard port on which applications submit their outgoing email with authentication, with STARTTLS encrypting the connection.
Port 25 is meant for traffic between mail servers and is blocked for outbound traffic by many providers and networks.
Detailed explanation →MTA #
An MTA (Mail Transfer Agent) is the server software that accepts email, queues it and passes it to the next server.
The MTA decides on retries after temporary errors and generates the error messages that reach the sender as a bounce.
ESP #
An ESP (Email Service Provider) is a service that sends email on behalf of customers, providing the infrastructure, reputation management and reporting.
The term covers both campaign platforms and pure sending services; the distinction is whether an editing and list-management environment is included.
Open tracking #
Open tracking records that an email was opened, by loading a small invisible image file when the message is displayed.
Since mailbox providers started preloading images to shield the recipient, an open is no longer reliable proof that anyone read the message.
Click tracking #
Click tracking records which links in an email are clicked, by routing those links through an intermediate URL.
Clicks are a stronger signal than opens because they require a deliberate action. They also count as a sign of engagement with mailbox providers.
Detailed explanation →Read on
A definition tells you what a term means, not what to do with it. These three pages cover the subjects in this list as a whole: the three authentication records, delivery in its entirety, and the practical steps of an actual send.
